As a free player of the English version you will currently obtain a total of 43 easily obtained Sacred Crystals from 3 all story missions and daily missions currently. Obtain these Sacred Crystals should take you at least 10 days which will give you 2 more Sacred Crystals from stamp card. Now that you have these 45 Sacred Crystals you have a choice to make. You can skip this next step in favor of getting to the point you can get the units you truly wish faster, or you can do the next step and potentially make event maps(As well as other maps) easier which will become your primary source of Sacred Crystals from this point onward.
Made your choice? Good!
Now assuming you choose to make events easier instead of getting to units you want faster. You will now go on your first premium roll spree. Assuming Aigis does not hate your shirt that day(aka the average) you will obtain from your 9 rolls... 4-5 Silver units, 3-4 Gold units, and 0-1 Platinum units. You are extremely lucky if you got a black unit out of these rolls. However your gold and platinum(if you got one) will serve you well. If you got Christopher in this group of silvers keep him. You'll need him later, and we do not have easy access to him.
(Note: If you do not mind remaking your account roll when you get 5-20 SC. If you do not get units to your liking remake the account and do it all over again until you do.)
Now that this initial boost of units is done we shall begin the period of saving. You will want to save a total of 100 SC that you get from events and the stamp card. Know that you will want to stay as close to this 100 SC as possible. If you are ever under this amount save until you are back to it. Now there is a reason for this madness. There are two types of events that will require you spend SC to get a perfect unit from it. Unless you are just a lucky bastard. In which case go play the lotto this guide is now invalid for you. These two event types are farm and collection. Farm requires you farm a map for the event unit to drop. While collection requires you to farm the event maps for a specific item. The more of this item you get the better the event unit will be when you receive it at the end of the event.
However, this is not why I am having you stockpile such a vast amount of SC. Only 40-50 of those SC are for events. The other 50-60 are for premium rolls at specific points in time. Now for what I'll have you do next I suggest you go read my unit class guide. This will give you knowledge of what a class does, and will help you in deciding which of those classes you actually wish to use. After understanding the different class(what we have now, and what we will have in the future) go to these two links.
Now include every unit class and choose up to 10 platinum and up to 10 gold units you'd eventually like to pull out of summons(Note: The ones with stars next to their names are gotten from outside of summons). The reason you are not allowed to choose a black unit is do to their rarity. No amount of preparation or timing will ever ensure you get what you want from there. Now when one of these twenty units are on the summon spotlight. You will roll 50-60 SC(10-12 rolls) to attempt to get them. This will give you the best chance of obtaining the premium unit you wanted. If you happen to get the unit you wanted out of the first few rolls feel free to either save(for the next spotlight) or spend(to get another copy to CR and SU) the rest of the SCs. Afterwards get back to that magical 100 SC point. However, note that you will still roll on that units next spotlight. Your goal is to get them -3 cost and 5/5 skill.
You may be wondering 'what do I do with the extra SC I get when I am at the 100 SC point already? Do I just save them?' The answer for now is no. You want to spend that excess on box expansion in the shop. It is one SC per 4 space. You can buy this a total of 35 times. Once you buy all the expansions you can. You should get access to the 'second barracks'(Not yet implemented). It is simply a storage for units you do not plan to use immediately(you cannot interact with units in the second barracks until they are taken out). You will save up the 30 SC to buy this as well. This is important due to the fact the longer you play the more silver, gold, fairies, armor, CR/SU fodder, and etc you'll have taking up all your space. So you'll need a lot of room.
After completing your storage expansion you now have one of three options. The first is that every time you get 5 extra SC you take your chances and premium roll; you save them until a spotlight of one of the selected twenty you choose to increase your chance of getting them; and finally you now select five black units you are interested in, and that excess goes to their spotlight whenever they happen to get one if at all. You can do the second and third options in tandem first using your 'black' SC during a gold/platinum spotlight before going into the reserve for that specifically.
Here is the link to decide on your five priority black units.
For those of you who wish to be more 'certain' on what you are going to get out of your SC. I would recommend completely skipping ever doing premium rolls after the initial few(Those few really help). This way you only need to have 40 SC stockpiled. You get to your 'needed' point faster, Can expand your 'barracks' sooner, and generally be less stressed. Using what extra you get for stamina/charisma refills for Thursday daily to get trading post units faster.